Methods: A group of 100 patients with documented sinus node dysfunction and 100 age- and sex- matched healthy control patients will be enrolled into this study. Gene polymorphism study includes the angiotensin-I converting enzyme gene, angiotensin II type 1 receptor gene, HCN1-4, SCN5A, KCNE and other possible gene polymorphisms which are related to sinus node function based on previous reports. Genetic polymorphisms are identified with polymerase chain reaction-based restriction analysis. Comparison of multiple haplotype analysis and single-locus analysis will be analyzed between the cases and controls.